An inkjet printing device is an example of a fluid ejection device that provides drop-on-demand (DOD) ejection of fluid droplets. In conventional DOD inkjet printers, printheads eject fluid droplets (e.g., ink) through a plurality of nozzles toward a print medium, such as a sheet of paper, to print an image onto the print medium. The nozzles are generally arranged in one or more arrays, such that properly sequenced ejection of ink from the nozzles causes characters or other images to be printed on the print medium as the printhead and the print medium move relative to one other.
One example of a DOD inkjet printer is a thermal inkjet (TIJ) printer. In a TIJ printer, a printhead includes a resistor heating element in a fluid-filled chamber that vaporizes fluid, creating a rapidly expanding bubble that forces a fluid droplet out of a printhead nozzle. Electric current passing through the heating element generates the heat, vaporizing a small portion of the fluid within the chamber. As the heating element cools the vapor bubble collapses, drawing more fluid from a reservoir into the chamber in preparation for ejecting another drop through the nozzle.
Unfortunately, thermal and electrical inefficiencies in the firing mechanism of the TIJ printhead (i.e., super-heating the fluid to form a vapor bubble) present a number of disadvantages that increase costs and reduce overall print quality in TIJ printers. One disadvantage, for example, is a decrease in firing performance over the life of the inkjet pen caused by a buildup of residue (koga) on the firing surface of the resistor heating element. Another disadvantage, when increasing the rate of drop ejection or firing speed (e.g., to increase image resolution while maintaining printed page throughput), is that the printhead can overheat, causing a vapor lock condition that prevents further firing and potential damage to the printhead. Another disadvantage is that the large electronic devices and power busses that drive thermally inefficient resistor heating elements take up costly silicon space in the TIJ printhead.